New Publication, Maps Show Depth of Bedrock Beneath the Ogallala Aquifer

LAWRENCE--The amount of ground water available in western Kansas can now be more accurately estimated, based on a recent report from the Kansas Geological Survey based at the University of Kansas.

The new publication provides a more precise picture of the thickness of the Ogallala aquifer, the source of nearly all the water used in western Kansas for irrigation, drinking, recreation, and industry.

The book, by Survey water scientists Allen Macfarlane and Brownie Wilson, features maps that show the top of the region's bedrock. Bedrock here consists of layers of chalk, shale, limestone, or sandstone that lie beneath the porous Ogallala aquifer and block the downward flow of ground water.

By outlining the top of the bedrock, the publication provides data that can be used to help determine the thickness of the Ogallala aquifer, which is composed of sand, gravels, and other rocks that are not naturally cemented together. The saturated thickness of the aquifer--the distance from bedrock to the top of the water table--is one indicator of how much water the aquifer may hold.

"Knowing saturated thickness is important for irrigators, for example, because they want to put a well in the deepest part of the aquifer," Macfarlane said. "Not knowing where the bedrock is, one farmer may drill a well above a high spot in the bedrock and not get much water while a neighbor's well above a low point in the bedrock would produce much longer."

In Kansas the depth of bedrock beneath the aquifer may vary from as much as 750 feet below the surface up to ground level.

Two large wall-sized maps showing bedrock elevations--one for northwest Kansas and the other for southwest Kansas--are included with the publication. One inch on the maps equals 4-3/4 miles. Until these maps were created, the most current map of the bedrock was based on data collected from wells drilled prior to the early 1980s. The new maps combine that earlier data with information from 22,535 more drilling records of water, oil, and gas wells that have become available over the past 25 years.

"Most of the changes on the new maps are in southwest Kansas, and some of them are dramatic," said Macfarlane.

The changes are most apparent in the details rather than the overall pattern of the depth of the bedrock across the region, he said. That is, the new data have filled in gaps where bedrock elevations were previously unknown, making it easier to determine if a significant amount of water is likely to be found in a specific location.

Ground-water management districts in the region will be able to use the data when developing tailored management plans and strategies for specific areas, Macfarlane said. Determining the depth of the bedrock also gives scientists a better understanding of the geologic history of the state.

"Enhancement of the Bedrock-Surface-Elevation Map Beneath the Ogallala Portion of the High Plains Aquifer, Western Kansas" is available from the Kansas Geological Survey, 1930 Constant Ave., Lawrence, KS 66047-3726 (or phone 785-864-3965). The cost is $15, plus $4 for handling and postage. Kansas residents should call for specific sales tax owed. More information about KGS books, maps, and other products is available at the Survey's web site (www.kgs.ku.edu).